GIA Spots Moissanite Posing as Rough Diamond

RAPAPORT... The Gemological Institute of America (GIA) has identified asynthetic moissanite stone that had been submitted to one of its laboratories as anatural rough diamond.The GIA's Carlsbad lab recently examined the 9.71-caratstone, whose shape was similar to an octahredron, staff gemologist RebeccaTsang wrote in the Winter 2017 edition of the institute's journal, Gems& Gemology. "While there is no evidence that this specimen wasdeliberately deceptive, gemological knowledge is important in preventingaccidental and intentional acts of fraud and preserving the integrity of thetrade," Tsang noted. Several physical traits of the piece of rough made it hardfor experts to identify it as a fake. These included a dimpled surface, as wellas lines that resembled a genuine rough diamond's growth features. In addition,the uneven surface made it difficult to reach the parts of the stone - such asinclusions - that would indicate its origins, Tsang wrote. In the end, the moissanite's "dichroism" - the appearance oftwo distinct colors under special light - showed the stone had a doublyrefractive nature. Unlike diamonds, which are only singly refractive,moissanite splits light rays in two, and has a higher refractive index. The researchers could tell it was synthetic, partly because natural moissaniteis extremely rare, having never been found in gem-quality form, Tsangexplained. When the material does occur naturally, it usually comes in muchsmaller pieces - which was, of course, not the case with this 9.71-carat stone. The GIA has publicized several discoveries of synthetic stonesdisguised as natural diamonds, including, most recently, a laboratory-grown polished diamond possessing a forged GIA inscription.Last year, two topaz stones, weighing 50.08 carats and 38.18 carats, were submitted to HRD Antwerp as rough diamonds. The stones were manipulated to resemble rough diamonds' growth features, the organization claimed.Image: Rebecca Tsang/GIA